The present invention relates to a composite filter for vehicular communication employing surface acoustic waves, in particular relates to a two-port surface acoustic wave resonant filter utilizing longitudinal resonance mode coupling disposed on one substrate and having different passbands.
In RF SAW filters being used in cellular telephones and portable telephones, ones that have passbands thereof mainly in the frequency band of from several hundreds MHz to several GHz are used.
Frequencies of SAW filters and necessary passbands thereof are decided according to systems to be incorporated and in general the specific bandwidths of several % are required.
The SAW filters of aforementioned usage are also necessary to be low in insertion loss. For example, ladder filters connected surface acoustic wave resonators in a ladder as disclosed in Japanese Patent Laid-open Application (KOKAI) No. HEI 5-183380, or longitudinal mode-coupled resonant filters in which a plurality of interdigital transducers (hereinafter refer to as IDT) are sandwiched by reflectors, as disclosed in Japanese Patent Laid-open Application (KOKAI) No. HEI 4-207615, have been mainly used. Further, these have been frequently used in combination (for instance, Japanese Patent Laid-open Application (KOKAI) No. HEI 8-65097). In any constitutions thereof, filters of relatively low loss can be constituted.
In applying these SAW filters in RF filters for portable telephones, passband widths that can be obtained depend largely on electromechanical coupling factors (k2) of piezoelectric substrates thereon the filters are formed. Accordingly, the piezoelectric substrates such as 36xc2x0 Y-X LiTaO3, 64xc2x0 Y-X LiNbO3, or 41xc2x0 Y-X LiNbO3 and so on of relatively large electromechanical coupling factor have been largely used.
In many of these piezoelectric substrates, when the thickness of a metal film formed thereon is increased, the conversion loss from surface acoustic waves to bulk waves increases, resulting in an increase of the insertion loss of the filter formed with that thickness.
When the thickness of a metal film is made thin, the electric resistance of an electrode formed on a piezoelectric substrate increases, resulting in an increase of the insertion loss.
As the result of these, in these piezoelectric substrates, a normalized film thickness of approximately several % (from approximately 3 to approximately 8%) becomes the best value, the normalized film thickness being one that is normalized by a wavelength of a propagating surface acoustic wave.
RF filters for portable telephones are being adopted in the following places.
One is a portion called a duplexer (transmission/reception switching element) that sends signals from an antenna to a receiving circuit or signals from a transmitting circuit to an antenna.
In a receiving stage, a RF filter is used as a receiving filter that after the signals from an antenna are amplified through a low noise amplifier, eliminates unnecessary signals. Similarly in a transmitting circuit, the RF filter is used as a filter of a transmitting stage.
In the receiving stage, the signals passed through the receiving filter are further mixed with signals from another high-frequency oscillator and are further sent to an intermediate-frequency filter. At this time, also to the signals from the aforementioned high-frequency oscillator, the RF filters are used.
So far, in such portable telephones, except for the duplexer, one filter is used for one passband. There are, however, cases where filters containing a plurality of passbands in one filter are desired.
For instance, there is a case when one filter is demanded to correspond to a plurality of portable telephone systems.
A second case is that the frequency band that has been used is made broader due to an increase of subscribers and a new band exists in distanced frequency from the original one. Even in this case, for all one system, the filter is required to have characteristics as if corresponding to two systems.
In still another case, upon forwarding small size of portable telephones, a compact filter in which a plurality of filters are accommodated together in one package is demanded.
Thus, there are strong demands for composite surface acoustic wave elements in which a plurality of filter characteristics are contained in one package.
Such composite filters are required further to be smaller. On the other hand, in order to constitute a smaller composite filter with surface acoustic wave filters there are the following problems.
First, from a viewpoint of smaller size, it is desirable to form a plurality of surface acoustic filters on a single piezoelectric substrate. Even when considered manufacturing processes, it is desirable to form, through filming and patterning of one conductive film, patterns of IDTs and reflectors from which a plurality of filters are formed.
However, in a composite filter, it is necessary to form a plurality of filters having passbands in different frequencies. A two-port surface acoustic wave resonant filter is liable to be affected on performance in particular by the normalized thickness. Accordingly, in the composite filter in which the two-port surface acoustic wave resonators are used, unless two frequency bands are adjacent (for instance, approximately 80 MHz apart in the neighborhood of 800 MHz), the filter with a single film thickness has been formed with difficulty.
In the case of the frequency bands being distanced more than the above, it is necessary to implement partial additional etching to vary a film thickness of an electrode for each constituting part of a filter. Accordingly productivity of the surface acoustic wave elements is deteriorated.
The present invention has been made in view of the problems mentioned above.
An object of the present invention is to provide a surface acoustic wave element having a structure that is low in insertion loss, is excellent in attenuation outside the band, can correspond to a plurality of systems or a plurality of frequency bands, is compact, and is high in productivity.
In order to accomplish the aforementioned object, a surface acoustic wave element of the present invention is provided with the following constituent elements and conditions.
A surface acoustic wave element of the present invention comprises a piezoelectric substrate, a first surface acoustic wave resonant filter, and a second surface acoustic wave resonant filter. The first surface acoustic wave resonant filter has a first IDT group consisting of a conductive film disposed on the piezoelectric substrate, and a first passband. The second surface acoustic wave resonant filter has a second IDT group consisting of the conductive film disposed on the piezoelectric substrate, and a second passband higher in frequency than that of the first passband. Here, the number of IDTs constituting the first IDT group is more than that of the IDTs constituting the second IDT group.
By constituting thus, a lower frequency filter necessary for a larger specific bandwidth is increased in kinds of higher modes that can be taken out. Thereby, the lower frequency filter is formed in a structure more appropriate for a broader band.
A surface acoustic wave resonant filter of higher frequency side is easy to make high attenuation in the passband of a surface acoustic wave resonant filter of lower frequency side. Thereby, when two surface acoustic wave resonant filters are connected, the insertion loss of the lower frequency filter can be prevented from increasing.
As the first surface acoustic wave resonant filter and the second surface acoustic wave resonant filter, a two-port surface acoustic wave resonant filter utilizing longitudinal mode coupling, for instance, can be cited.
Thereby, two-port surface acoustic wave resonant filters utilizing longitudinal mode coupling having passbands of different frequencies can be combined to correspond to a plurality of systems or a plurality of frequency bands.
Grounding electrodes of IDTs constituting the first IDT group are disposed electrically independent from each other on the piezoelectric substrate. The grounding electrodes of at least two aforementioned IDTs disposed in an excitation direction of surface acoustic waves among the second IDT group are connected on the piezoelectric substrate. Further, the first surface acoustic wave resonant filter and the second surface acoustic wave resonant filter can be made to have an output terminal connected in common on the piezoelectric substrate.
The bandwidth of the first passband can be set approximately equal to or broader than the bandwidth of the second passband.
The first passband may be set in the range of, for instance, approximately 720 MHz and approximately 745 MHz, and the second passband can be set in the range of, for instance, approximately 930 MHz and approximately 960 MHz.
Further, the first passband may be set in the range of, for instance, approximately 810 MHz and approximately 830 MHz, and the second passband can be set in the range of, for instance, approximately 870 MHz and approximately 887 MHz.
Still further, the first passband may be set in the range of, for instance, approximately 889 MHz and approximately 898 MHz, and the second passband can be set in the range of, for instance, approximately 925 MHz and, approximately 960 MHz.
The passbands shown here are examples of the passbands to which a surface acoustic wave element of the present invention is applicable. And other passbands than these can be also applicable. For instance, in the European GSM system, a center of the first passband is set at 732.5 MHz and a center of the second passband may be set at 947.5 MHz.
As the piezoelectric substrate constituting the surface acoustic wave element of the present invention, for instance, 36xc2x0 Y-cut X-transmission LiTaO3, 64xc2x0 Y-cut X-transmission LiNbO3, or 41xc2x0 Y-cut X-transmission LiNbO3 and so on may be used. The piezoelectric substrates other than these may be used.
These substrates have relatively large electromechanical coupling factors and are easy in selection of an appropriate pass bandwidth.
The surface acoustic wave element of the present invention is characterized in that the first surface acoustic wave resonant filter and the second surface acoustic wave resonant filter are disposed on the piezoelectric substrate along a direction approximately perpendicular to a direction of excitation of the surface acoustic waves.
By adopting such a constitution, the number of reflection electrodes constituting the reflectors can be reduced to make, for instance, smaller the size of the surface acoustic wave element. Even in that case, the surface acoustic waves leaked from one surface acoustic wave resonant filter can be suppressed in affecting to the other surface acoustic wave resonant filter to the minimum level.
A pitch of electrode fingers (distance between a central line of an electrode finger and that of an adjacent electrode finger) of the IDTs of a low passband side filter is formed larger than that of the IDTs of a high passband side filter.
The number of the IDTs constituting the first IDT group is selected from 3, 5, 7 and 9 and is more than that of the IDTs constituting the second IDT group.
The effective and practical combination of the number of the IDTs constituting the first IDT group and that of the IDTs constituting the second IDT group is preferable to be 5:3, 7:3, 7:5, 9:5 and 9:7.
A surface acoustic wave device of the present invention comprises a surface acoustic wave element and an package mounting the surface acoustic wave element. The surface acoustic wave element comprises a piezoelectric substrate, a first surface acoustic wave resonant filter, and a second surface acoustic wave resonant filter. The first surface acoustic wave resonant filter has a first IDT group consisting of a conductive film disposed on the piezoelectric substrate and has a first passband. The second surface acoustic wave resonant filter has a second IDT group consisting of the conductive film disposed on the piezoelectric substrate and has a second passband higher in its frequency than the first passband. Here, the number of IDTs constituting the first IDT group is more than that of the IDTs constituting the second IDT group. The package that mounts the surface acoustic wave element comprises a first input terminal to the first surface acoustic wave filter, a second input terminal to the second surface acoustic wave filter, and an output terminal from the first surface acoustic wave filter and the second surface acoustic wave filter. Here, the first input terminal and the second input terminal, and the output terminal are disposed in a direction approximately parallel with a direction of excitation of the surface acoustic waves of the surface acoustic wave element so as to face each other the surface acoustic wave element therebetween.
The aforementioned two surface acoustic wave filters are electrically connected by use of wire bonding to bonding portions formed on an package accommodating the piezoelectric substrate on which the two filters are formed. In addition to this, two filters are disposed to have a propagating direction of the surface acoustic waves in a direction from the input terminal to the output terminal of terminals of the package and are disposed in a direction approximately perpendicular to the propagating direction of surface acoustic waves.
Two filters are disposed in an approximately perpendicular direction to the propagating direction of the surface acoustic waves. Therefore, even when the number of reflection electrodes of the reflector is reduced for smaller size purpose, leakage surface acoustic wave of one filter hardly affects on the other filter. Since the bonding wires of the input/output can be disposed without crossing the IDTs of the two filters, without disturbing the attenuation out of band, an excellent surface acoustic wave element can be obtained.